Chapter 25
25.1 Plant Nutrition and Soil
1. The ancient Greeks considered plants “soil-eaters” that converted soil into plant
tissue.
2. The 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment.
a. He planted a five pound young willow tree in a pot with 200 pounds of soil.
b. After five years of watering, the tree weighed 170 pounds but only a few ounces
of soil was missing.
c. He concluded the increase in tree weight came from water; he was unaware of
substances in air.
A. Essential Inorganic Nutrients
1. Approximately 95% of a plant’s dry weight is carbon, hydrogen, and oxygen.
a.
Carbon dioxide is the source of carbon for a plant.
b.
Water is the source of hydrogen.
c. Oxygen can come from either atmospheric oxygen, carbon dioxide, or water.
2. A mineral is an organic substance usually containing two or more elements.
3. To be classified as an essential nutrient, the following criteria must be fulfilled.
a. It must have an identifiable nutritional role.
b. No other element can substitute and fulfill the same role.
c. A deficiency of the element causes the plant to die.
4. These elements are divided into macronutrients and micronutrients, according to
their relative concentrations in plant tissue.
5. Beneficial nutrients are elements required for or to improve growth of a particular
plant.
a. Horsetails require silicon as a mineral nutrient.
b. Sugar beets show better growth in the presence of sodium.
c. Soybeans use nickel when root nodules are present.
B. Determination of Essential Nutrients
1. When a plant is burned, most mineral elements (except for nitrogen) remain in the
ash.
2. Hydroponics (water culture) is the preferred method for determining plant mineral
requirements.
a. Hydroponics is cultivation of plants in water.
b. Nutrient requirements of plants are determined by omitting a mineral and
observing the effects.
c. If plant growth suffers, it can be concluded that the omitted mineral is a required
nutrient.
d. This works for macronutrients but impurities make micronutrient measurement
difficult.
C. Hydroponics
1.
Some of the benefits of hydroponics is the elimation of plant pests, diseases, and
weeds.
2. Since water is used through a pipline system, there is little water lost through runoff,
compared with traditional irrigation methods.
D. Soil
1. Soil Formation
a.
Soil formation begins with weathering of rock by freezing, glacier flow, stream
flow, and chemicals.
b. Lichens and mosses grow on barren rock and trap particles and leave decaying
tissues.
c. Decayed organic matter (humus) takes time to accumulate; its acidity leaches
minerals from rocks.
d. Depending on parent material and weathering, a centimeter of soil may develop
within 15 years.
2. The Nutritional Function of Soil
a. Soil consists of mineral particles, decaying organic matter, living organisms, air
and water.
b. The best soil includes particles of different sizes; this provides critical air spaces.
c. Mineral Particles
1) Mineral particles vary in size.
a) Sand particles are largest: 0.05–2.0 mm in diameter.
b) Sand particles are medium sized: 0.002–0.05 mm in diameter.
c) Clay particles are smallest: below 0.002 mm in diameter.
2) Sandy soils lose water too readily; clay packs tightly to hold water.
3) Clay particles are negatively charged and attract positively charged ions (e.g.,
calcium [Ca2+] and potassium [K+]).
4) In acidic soils, hydrogen ions replace positively charged nutrients and the
nutrient ions float free and are leached; this is why acid rain kills trees.
5) Clay cannot retain negatively charged NO3 , and the nitrogen content of clay
soil is low.
6) Loam (a mixture of the three mineral particles) retains water and nutrients;
roots take up oxygen in the air spaces.
d. Humus
1) A mixture of 10–20% humus mixed with a top layer of soil particles is best
for plants.
2) Humus keeps soil loose and crumbly, decreases runoff and aerates soil.
3) Humus is acidic and retains positively charged minerals for plants to use later.
4) Bacteria and fungi break down organic matter in humus and return inorganic
nutrients to plants.
e. Living Organisms
1) Small plants play a major role in the formation of soil from rock and in
succession.
2) Roots of larger plants penetrate the soil and weather rocks.
3) Larger moles and badgers and smaller earthworms help turn over the soil.
4) Soil animals, from mites to millipedes, help break down leaves and other plant
remains.
5) Fungi, protozoa, algae and bacteria complete decomposition.
6) Soil bacteria make nitrate available to plants.
7) Some soil organisms (i.e. some roundworms) are crop pests that feed on roots.
3. Soil Profiles
a. A soil profile is a vertical section from the ground surface to the unaltered rock
below; usually, a soil profile has parallel layers called horizons.
1) The A horizon is the uppermost topsoil layer that contains litter and humus.
2) The B horizon lacks organic matter but contains inorganic nutrients leached
from the A horizon.
3) The C horizon is weathered and shattered bedrock.
4) Soil profiles vary by parent material, climate and ecosystem.
5) Grassland soils have deep A horizons from turnover of decaying grasses and
lack of leaching.
6) Forest soils have thinner A horizons but enough inorganic nutrients for tree
root growth.
7) Tropical rain forest A horizons are shallow due to rapid decomposition; the B
horizon is deeper due to extensive leaching.
4. Soil Erosion
a. Soil erosion is caused by water or wind carrying away soil.
b. Erosion removes 25 billion tons of topsoil worldwide annually.
c. Deforestation and desertification contribute to erosion.
d. U.S. farmlands lose soil faster than it is formed on one-third of cropland.
e. The coastal wetlands are also losing soil at a high rate.
25.2 Water and Mineral Uptake
1. Minerals follow the path of water uptake.
a. Some mineral ions diffuse in between the cells; this is called apoplastic transport.
b. Because of the impermeable Casparian strip (a band of suberin and lignin
bordering four sides of root endodermal cells), water must eventually enter the
cytoplasm of endodermal cells.
c. Water can enter epidermal cells at their root hairs, and progress across the cortex
and endodermis of a root.
A. Mineral Uptake
1.
In contrast to water, minerals are actively taken up by plant cells.
a. Mineral nutrient concentration in roots may be 10,000 times more than in
surrounding soil.
b. During transport throughout a plant, minerals can exit xylem and enter cells that
require them.
2. Mineral ions cross plasma membranes by a chemiosmotic mechanism.
a. Plants absorb minerals in ionic form: nitrate (NO3 ), phosphate (HPO4= ), and
potassium ions (K+); all have difficulty crossing a charged plasma membrane.
b. It has long been known plants expend energy to actively take up and concentrate
mineral ions.
c. A plasma membrane pump, called a proton pump, hydrolyzes ATP to transport H+
ions out of the cell; this sets up an electrochemical gradient that causes positive
ions to flow into cells.
d. Negative ions are carried across the plasma membrane in conjunction with H+
ions as H+ ions diffuse down their concentration gradient.
B. Adaptations of Roots for Mineral Uptake
1. Two symbiotic relationships are known to assist roots in acquiring nutrients.
2. Legumes have roots colonized by the bacterium Rhizobium.
a. Plants cannot use atmospheric nitrogen because they lack enzymes to break the N
d.
b. Rhizobium makes nitrogen compounds available to plants in exchange for
carbohydrates.
c. Bacteria live in root nodules—structures on plant roots that contain
nitrogen-fixing bacteria.
d. Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+)
(nitrogen-fixation).
e. Other plants have a relationship with free-living, nitrogen-fixing microorganisms
in soil.
3. Most plants have mycorrhizae; those lacking mycorrhizae are limited in where they
can grow.
a. Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant
roots.
b. The fungal hyphae may enter the cortex of roots but do not enter plant cells.
c. Ectomycorrhizae form a mantle exterior to the root, and they grow between cell
walls.
d. Fungus increases the surface area for mineral and water uptake and breaks down
organic matter.
e. In return the root furnishes the fungus with sugars and amino acids.
f. Orchid seeds are small with limited nutrients; they germinate only when invaded
by mycorrhizae.
g. Nonphotosynthetic plants (e.g., Indian pipe) use mycorrhizae to extract nutrients
from nearby trees.
4. Some plants have poorly developed roots or no roots; other mechanisms supply
minerals and water.
a. Parasitic plants (e.g., dodders, broomrapes, pinedrops) send out haustoria (rootlike
projections) that grow into the host and tap into the xylem and phloem of the host.
b. Venus’s-flytrap and sundew obtain nitrogen and minerals as leaves capture and
digest insects.
25.3 Transport Mechanisms in Plants
1. Flowering plants have transport tissues as an adaptation to living on land: xylem and
phloem.
A. Reviewing Xylem and Phloem Structure
2. Xylem transport tissue conducts water and mineral solutes from roots to leaves; it
contains two types of conducting cells: tracheids and vessel elements.
a. Tracheids:
1) are hollow, nonliving cells with tapered overlapping ends;
2) are thinner and longer than vessel elements; and
3) water crosses the end and sidewalls because of pits in secondary cell wall.
b. Vessel elements:
1) are hollow, nonliving cells that lack tapered ends;
2) are wider and shorter than tracheids;
3) lack transverse end walls; and
4) form a continuous pipeline for water and mineral transport.
3. Phloem is vascular tissue that conducts organic nutrients to all parts of the plant; it
contains sieve-tube members and companion cells.
a. Sieve-tube members lack a nucleus, are arranged end to end and have channels in
end walls (thus, the name “sieve-tube”) through which plasmodesmata extend
from one cell to another.
b. Companion cells connect to sieve-tube cells by numerous plasmodesmata, and are
smaller and more generalized than sieve-tube cells; they have a nucleus.
B. Determining Xylem and Phloem Function
1. The transport systems of xylem and phloem rely on the mechanical properties of
water.
a. Diffusion moves molecules from higher to lower concentrations.
b. Water potential considers both water pressure and osmotic pressure.
2. They also rely on the chemical properties of water: polarity of water and hydrogen
bonding.
C. The Concept of Water Potential (Science Focus box)
1. Water potential is the potential energy of water.
a. For example, water at the top of a waterfall has a higher water potential than
water at the bottom of the waterfall.
2. In cells, two things determine water potential:
a. Water pressure across a membrane
1) Within water pressure, pressure potential is the effect that pressure has on
water potential.
2) Water moves from an area of higher pressure to an area of lower pressure.
3) The higher the water pressure, the higher the water potential.
4) Pressure potential is the concept that best explains the movement of sap in
xylem and phloem
b. Solute concentration across a membrane
1) Osmotic potential takes into account the effects of solutes on the movement of
water.
2) Water tends to move across a membrane from the area of lower solute
concentration to the area of higher solute concentration.
3) The lower the concentration of solutes (osmotic potential), the higher the
water potential.
3. Turgor pressure is the pressure potential that increases due to the process of osmosis.
a. Water stops entering a cell when the pressure potential inside the cell increases
and balances the osmotic potential outside the cell.
b. Plants depend on turgor pressure to maintain the turgidity of their bodies.
c. When plants do not have sufficient turgor pressure, the plant droops as a result.
D. Water Transport
1. Movement of water and minerals in a plant involves entry into roots, xylem, and
leaves.
2. Water and minerals enter root cells before they reach xylem by the two routes already
described.
3. Water entering root cells creates a positive pressure called root pressure.
a. Root pressure (which occurs primarily at night) tends to push xylem sap upward
in a plant.
b. Guttation is the appearance of drops of water along the edges of leaves, as a
result of water being forced out of leaf vein endings; it is the result of root
pressure.
c. Root pressure is not a sufficient mechanism for water to rise to the tops of trees.
4. Cohesion-Tension Model of Xylem Transport
a. Water and dissolved minerals must be transported upward from roots to xylem,
perhaps as high as 90 meters.
b. The cohesion-tension model states that transpiration creates a tension (i.e., a
negative pressure) that pulls water upward in xylem without expending any
energy of the plant.
c. Water molecules are cohesive with one another, adhesive with xylem walls.
d. Water molecules interact with one another and form a continuous water column
in the xylem, from the leaves to the roots that is difficult to break.
e. Transpiration is a plant’s loss of water to the atmosphere through evaporation at
leaf stomata.
f. Cohesion is the tendency of water molecules to cling together due to their
forming hydrogen bonds.
g. Adhesion is the ability of water (a polar molecule) to interact with molecules
comprising the walls of xylem vessels; adhesion gives a water column extra
strength and prevents it from slipping back down.
h. In daytime, the negative water potential created by transpiration extends from
leaves to roots; the water column must be continuous.
i. If a water column within xylem is broken by cutting a stem, the water column will
drop back down the xylem vessel away from the site of breakage, making it more
difficult for conduction to occur.
j. At least 90% of the water taken up by roots is lost through stomata by
transpiration.
k. With plenty of water, stomata will remain open, allowing CO2 to enter the leaf
and photosynthesis to occur.
l. Transpiration exerts a pulling force or tension that draws the water column up in
vessels.
m. Under water stress, more water is lost through a leaf than can be brought up and
the stomata close; the leaves are then protected from water loss by the waxy
cuticle of the upper and lower epidermis.
n. Photosynthesis requires CO2 to enter the leaf; there must be sufficient water so
stomata can remain open and allow CO2 to enter.
o. In the root, water enters the xylem passively by osmosis.
p. The water column in the xylem extends from leaves to the root.
q. Water is pulled upward from the roots due to the tension in the xylem created by
the evaporation of water at the leaves.
E. Opening and Closing of Stomata
1. Each stoma, a small pore in the leaf epidermis, has two guard cells.
2. Stomata open from turgor pressure when guard cells take up water; when they lose
water, turgor pressure decreases and stomata close.
3. Guard cells are attached to each other at their ends; the inner walls are thicker than
outer walls.
4. Radial expansion is prevented by cellulose microfibrils in the walls but outer walls
can expand lengthwise.
5. As they take up water, they buckle out, thereby creating an opening between cells.
6. Since 1968, it has been known that when stomata open, there is accumulation of K+
ions in guard cells.
7. A proton pump run by breakdown of ATP to ADP and P transports H+ outside the
cell; this establishes an electrochemical gradient allowing K+ to enter by way of a
channel protein.
8. The blue-light component of sunlight is a signal that can cause stomata to open.
a. There is evidence that flavin pigments absorb blue light.
b. This pigment sets in motion a cytoplasmic response activating the proton pump
that causes K+ ions to accumulate in guard cells.
9. Evidence suggests a receptor in the plasma membrane of guard cells brings about
inactivation of the proton pump when CO2 concentration rises, as happens when
photosynthesis ceases.
10. Abscisic acid (ABA) produced by cells in wilting leaves, also causes stomata to close;
photosynthesis cannot occur, but water is conserved.
11. In plants kept in the dark, stomata open and close on a 24-hour basis as if responding
to sunlight in the daytime and the absence of sunlight at night; some sort of internal
biological clock must keep time.
F. Plant Can Clean Up Toxic Messes (Ecology Focus box)
1. Phytoremediation uses plants to absorb, store, degrade, or transform pollutants that
would otherwise harm the environment.
2. Plants can clean up contaniminated sites in one of two ways:
a. If the substance is an organic contaminant (i.e. spilled oil), the plants break down
the substances and the remainders can either be absorbed by the plant or left in the
soil or water.
b. If the substance is an inorganic contaminant (i.e. cadmium, zinc), the plants
absorb the substances, trap the contaminants and the plants are harvested and
disposed of.
3. Poplars Take Up Excessive Nitrates
a. A farmer in Amana, Iowa planted a mile long section of poplars to absorbe nitrate
runoff from a fertilized cornfield.
b. Before the trees were planted, nitrate levels were as much as ten times the amount
considered safe.
c. Following the tree planting, the brook’s nitrate levels dropped more than 90%,
and the trees have thrived.
4. Canola Plants Take Up Selenium
a. Canola plants were planted in farms in California’s San Joaquin Valley to absorb
excess selenium in the soil.
b. Irrigation can cause naturally occurring selenium to rise to the soil surface and the
selenium would flow off into drainage ditches and eventually in wildlife areas.
Selenium can deform and kill waterfowl.
c. By adding selenium-accumulating canola plants to farmers’ crop rotations,
selenium levels in runoff are being managed in the area.
5. Mustard Plants Take Up Uranium
a. In a Superfund site in Aberdenn, Maryland, mustard plants were used to remove
uranium from the Army’s firing range, for as little as 10% of the cost of
traditional clean up methods.
6. Limitations of Phytoremediation
a. One limitation of phytoremediation is the slow pace at which trees grow and
therefore, how much pollutants the trees can absorb each growing season.
b. A second limitation is that phytoremediation is only effective at depths that plant
roots can reach, making it useless against deep-lying contamination unless the
contaminated soils are excavated.
c. Another limitation is that phytoremediation is not successful at cleaning up lead
or other metal contaminatated areas without adding chemicals to the soil.
d. In addition, there is the risk that animals may ingest the pollutants by eating the
leaves of plants that are being used in phytoremediation.
G. Organic Nutrient Transport
1. Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots.
a. He observed the results of removing a strip of bark from a tree (girdling).
b. Bark swells just above the cut and sugar accumulates in the swollen tissue.
c. Today, we know phloem was removed but xylem remained; therefore, phloem
does transport sugars.
2. Radioactive tracer studies using 14C confirmed phloem transports organic nutrients.
a. When 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar
moves to roots.
b. Similar studies confirm phloem transports amino acids, hormones, and mineral
ions.
3. Aphids Used in Study
a. It is difficult to take samples of sap from just the phloem cells without injuring the
phloem.
b. Aphids (small insects) drive their mouth stylets into a sieve-tube cell; then
samples are easily taken.
c. The aphid body is cut off; the stylet becomes a small needle from which phloem
is collected.
d. Such research indicates that sap can move through phloem from 60–100 cm per
hour or more.
4. Pressure-Flow Model of Phloem Transport
a. The pressure-flow model explains the transport of sap through sieve tubes by a
positive pressure potential.
b. The buildup of water creates a positive pressure potential within the sieve tubes
that moves water and sucrose to a sink (e.g., at the roots).
c. Pressure exists from the leaves to the roots; at the roots, sucrose is transported out
and water also flows through due to the pressure.
d. Consequently, this pressure gradient causes a flow of water from leaves to roots.
e. The conducting cells of phloem are sieve tubes lined end to end.
f. Cytoplasm extends through the sieve plates of adjoining cells to form a
continuous tube system.
g. During the growing season, leaves produce sugar.
h. Sucrose is actively transported into phloem by an electrochemical gradient
established by a H+ pump.
i. Water flows passively into sieve tubes by osmosis.
j.
k.
A sink can be at the roots or any other part of the plant that requires nutrients.
Because phloem sap flows from source to sink, sap can move any direction along
phloem.